Chapter 9 Image Compression Standards 9.1 The JPEG Standard 9.2 The JPEG2000 Standard 9.3 The JPEG-LS Standard 9.5 Further Exploration Li & Drew1

Fundamentals of Multimedia, Chapter The JPEG Standard JPEG is an image compression standard that was developed by the “Joint Photographic Experts Group”. JPEG was formally accepted as an international standard in JPEG is a lossy image compression method. It employs a transform coding method using the DCT (Discrete Cosine Transform). An image is a function of i and j (or conventionally x and y ) in the spatial domain. The 2D DCT is used as one step in JPEG in order to yield a frequency response which is a function F(u, v) in the spatial frequency domain, indexed by two integers u and v. Li & Drew2

Fundamentals of Multimedia, Chapter 9 Observations for JPEG Image Compression The effectiveness of the DCT transform coding method in JPEG relies on 3 major observations: Observation 1: Useful image contents change relatively slowly across the image, i.e., it is unusual for intensity values to vary widely several times in a small area, for example, within an 8×8 image block. much of the information in an image is repeated, hence “spatial redundancy”. Li & Drew3

Fundamentals of Multimedia, Chapter 9 Observations for JPEG Image Compression (cont’d) Observation 2: Psychophysical experiments suggest that humans are much less likely to notice the loss of very high spatial frequency components than the loss of lower frequency components. the spatial redundancy can be reduced by largely reducing the high spatial frequency contents. Observation 3: Visual acuity (accuracy in distinguishing closely spaced lines) is much greater for gray (“black and white”) than for color. chroma subsampling (4:2:0) is used in JPEG. Li & Drew4

Fundamentals of Multimedia, Chapter 9 Fig. 9.1: Block diagram for JPEG encoder. Li & Drew5

Fundamentals of Multimedia, Chapter Main Steps in JPEG Image Compression Transform RGB to YIQ or YUV and subsample color. DCT on image blocks. Quantization. Zig-zag ordering and run-length encoding. Entropy coding. Li & Drew6

Fundamentals of Multimedia, Chapter 9 DCT on image blocks Each image is divided into 8 × 8 blocks. The 2D DCT is applied to each block image f(i, j), with output being the DCT coefficients F(u, v) for each block. Using blocks, however, has the effect of isolating each block from its neighboring context. This is why JPEG images look choppy (“blocky”) when a high compression ratio is specified by the user. Li & Drew7

Fundamentals of Multimedia, Chapter 9 Quantization (9.1) F(u, v) represents a DCT coefficient, Q(u, v) is a “quantization matrix” entry, and represents the quantized DCT coefficients which JPEG will use in the succeeding entropy coding. – The quantization step is the main source for loss in JPEG compression. – The entries of Q(u, v) tend to have larger values towards the lower right corner. This aims to introduce more loss at the higher spatial frequencies — a practice supported by Observations 1 and 2. – Table 9.1 and 9.2 show the default Q(u, v) values obtained from psychophysical studies with the goal of maximizing the compression ratio while minimizing perceptual losses in JPEG images. Li & Drew8

Fundamentals of Multimedia, Chapter 9 Table 9.1 The Luminance Quantization Table Table 9.2 The Chrominance Quantization Table Li & Drew

Fundamentals of Multimedia, Chapter 9 An 8 × 8 block from the Y image of ‘Lena’ Fig. 9.2: JPEG compression for a smooth image block. Li & Drew f(i, j) F(u, v) DCT Coefficients Original Image Block

Fundamentals of Multimedia, Chapter 9 Li & Drew11 Fig. 9.2 (cont’d): JPEG compression for a smooth image block. Quantized DCT Coefficients De-Quantized DCT Coefficients Reconstructed Image Block Error

Fundamentals of Multimedia, Chapter 9 Another 8 × 8 block from the Y image of ‘Lena’ Fig. 9.3: JPEG compression for a textured image block. Li & Drew f(i, j) F(u, v)

Fundamentals of Multimedia, Chapter 9 Li & Drew13 Fig. 9.3 (cont’d): JPEG compression for a textured image block.

Fundamentals of Multimedia, Chapter 9 Run-length Coding (RLC) on AC coefficients RLC aims to turn the values into sets {#-zeros-to-skip, next non-zero value}. To make it most likely to hit a long run of zeros: a zig-zag scan is used to turn the 8×8 matrix into a 64-vector. Fig. 9.4: Zig-Zag Scan in JPEG. Li & Drew14

Fundamentals of Multimedia, Chapter 9 DPCM on DC coefficients The DC coefficients are coded separately from the AC ones. Differential Pulse Code modulation (DPCM) is the coding method. If the DC coefficients for the first 5 image blocks are 150, 155, 149, 152, 144, then the DPCM would produce 150, 5, - 6, 3, -8, assuming d i = DC i+1 − DC i, and d 0 = DC 0. Li & Drew15

Fundamentals of Multimedia, Chapter 9 Entropy Coding The DC and AC coefficients finally undergo an entropy coding step to gain a possible further compression. Use DC as an example: each DPCM coded DC coefficient is represented by (SIZE, AMPLITUDE), where SIZE indicates how many bits are needed for representing the coefficient, and AMPLITUDE contains the actual bits. In the example we’re using, codes 150, 5, −6, 3, −8 will be turned into (8, ), (3, 101), (3, 001), (2, 11), (4, 0111). SIZE is Huffman coded since smaller SIZEs occur much more often. AMPLITUDE is not Huffman coded, its value can change widely so Huffman coding has no appreciable benefit. Li & Drew16

Fundamentals of Multimedia, Chapter Four Commonly Used JPEG Modes Sequential Mode — the default JPEG mode, implicitly assumed in the discussions so far. Each graylevel image or color image component is encoded in a single left-to-right, top-to-bottom scan. Progressive Mode. Hierarchical Mode. Lossless Mode — discussed in Chapter 7, to be replaced by JPEG-LS (Section 9.3). Li & Drew17

Fundamentals of Multimedia, Chapter The JPEG2000 Standard Design Goals: – To provide a better rate-distortion tradeoff and improved subjective image quality. – To provide additional functionalities lacking in the current JPEG standard. The JPEG2000 standard addresses the following problems: – Lossless and Lossy Compression: There is currently no standard that can provide superior lossless compression and lossy compression in a single bitstream. Li & Drew18

Fundamentals of Multimedia, Chapter 9 – Low Bit-rate Compression: The current JPEG standard offers excellent rate-distortion performance in mid and high bit-rates. However, at bit-rates below 0.25 bpp, subjective distortion becomes unacceptable. This is important if we hope to receive images on our web- enabled ubiquitous devices, such as web-aware wristwatches and so on. – Large Images: The new standard will allow image resolutions greater than 64K by 64K without tiling. It can handle image size up to 2 32 − 1. – Single Decompression Architecture: The current JPEG standard has 44 modes, many of which are application specific and not used by the majority of JPEG decoders. Li & Drew19

Fundamentals of Multimedia, Chapter 9 – Transmission in Noisy Environments: The new standard will provide improved error resilience for transmission in noisy environments such as wireless networks and the Internet. – Progressive Transmission: The new standard provides seamless quality and resolution scalability from low to high bit-rate. The target bit-rate and reconstruction resolution need not be known at the time of compression. – Region of Interest Coding: The new standard allows the specification of Regions of Interest (ROI) which can be coded with superior quality than the rest of the image. One might like to code the face of a speaker with more quality than the surrounding furniture. Li & Drew20

Fundamentals of Multimedia, Chapter 9 – Computer Generated Imagery: The current JPEG standard is optimized for natural imagery and does not perform well on computer generated imagery. – Compound Documents: The new standard offers metadata mechanisms for incorporating additional non-image data as part of the file. This might be useful for including text along with imagery, as one important example. In addition, JPEG2000 is able to handle up to 256 channels of information whereas the current JPEG standard is only able to handle three color channels. Li & Drew21

Fundamentals of Multimedia, Chapter 9 Properties of JPEG2000 Image Compression Uses Embedded Block Coding with Optimized Truncation (EBCOT) algorithm which partitions each subband LL, LH, HL, HH produced by the wavelet transform into small blocks called “code blocks”. A separate scalable bitstream is generated for each code block ⇒ improved error resilience. Fig. 9.7: Code block structure of EBCOT. Li & Drew22

Fundamentals of Multimedia, Chapter 9 (a) Fig. 9.13: Comparison of JPEG and JPEG2000. (a) Original image. Li & Drew23

Fundamentals of Multimedia, Chapter 9 (c) Fig (Cont’d): Comparison of JPEG and JPEG2000. (b) JPEG (left) and JPEG2000 (right) images compressed at 0.75 bpp. (c) JPEG (left) and JPEG2000 (right) images compressed at 0.25 bpp. Li & Drew24 (b)

Fundamentals of Multimedia, Chapter The JPEG-LS Standard JPEG-LS is in the current ISO/ITU standard for lossless or “near lossless” compression of continuous tone images. It is part of a larger ISO effort aimed at better compression of medical images. Uses the LOCO-I (LOw COmplexity LOssless Compression for Images) algorithm proposed by Hewlett-Packard. Motivated by the observation that complexity reduction is often more important than small increases in compression offered by more complex algorithms. Main Advantage: Low complexity! Li & Drew25

Fundamentals of Multimedia, Chapter JBIG and JBIG-2: Bi-level Image Compression Standards Main Goal: Enables the handing of documents in electronic form. Primarily used to code scanned images of printed or hand-written text, computer generated text, and facsimile transmissions. JBIG is a lossless compression standard. It also offers progressive encoding/decoding capability, the resulting bitstream contains a set of progressively higher resolution images. JBIG-2 introduces model-based coding – similar to context-based coding. It supports lossy compressions well. Li & Drew26